1. Field of the Invention
The present invention relates to a magnetic particle and to a method of preparing the same. More particularly, the present invention relates to a magnetic particle that has magnetic characteristics suited to magnetic recording and that can be employed in a particulate magnetic recording medium, and to a method of preparing the same.
The present invention further relates to a particulate magnetic recording medium comprising the above magnetic particle.
2. Discussion of the Background
Conventionally, ferromagnetic metal powders have been primarily employed in the magnet layers of high-density recording-use magnetic recording media. These ferromagnetic metal powders are comprised of acicular particles consisting of a main material primarily in the form of iron. Particle size reduction and increased coercive force are sought for high-density recording. These ferromagnetic metal powders are employed in magnetic recording media for various uses.
Since the quantity of information being recorded has been increased in recent years, high-density recording is constantly required of magnetic recording media. However, in achieving ever higher density recording, limits to the improvement in ferromagnetic metal powders have begun to appear. That is because as the particle size of a ferromagnetic metal powder decreases, thermal fluctuation ends up causing superparamagnetism, precluding use in magnetic recording media.
By contrast, hexagonal ferrite magnetic material has high crystal magnetic anisotropy due to its crystalline structure and thus exhibits good thermal stability. Therefore, even with size reduction, it is possible to maintain good magnetic characteristics suited to magnetic recording. However, in magnetic materials having high crystal magnetic anisotropy such as ferrite magnetic material, the switching magnetic field increases, resulting in high coercive force. This is problematic in that a large external magnetic field is required for recording, compromising recording properties.
For example, Japanese Patent No. 2,659,957, which is expressly incorporated herein by reference in its entirety, proposes reducing a substituted ferromagnetic ferrite powder in a hydrogen gas flow as a means of improving the recording properties of a hexagonal ferrite magnetic powder. Japanese Patent No. 2,659,957 describes reducing a substituted ferromagnetic ferrite powder in a hydrogen gas flow to obtain a magnetic powder of suitable coercive force and high saturation magnetization.
Additionally, the Journal of the Magnetics Society of Japan 29, 239-242 (2005), which is expressly incorporated herein by reference in its entirety, describes attempts that have been made to reduce the switching magnetic field by stacking a soft magnetic layer and a hard magnetic layer formed as vapor phase films on a nonmagnetic inorganic material to produce exchange coupling interaction.
In metal thin-film magnetic recording media such as HD media, a glass substrate capable of withstanding high temperatures during vapor deposition is normally employed as the support. By contrast, particulate magnetic recording media affording good general-purpose properties and employing inexpensive organic material supports have been proposed in recent years, and are widely employed as video tapes, computer tapes, flexible disks, and the like. Accordingly, in order to enhance recording properties when hexagonal ferrite magnetic material is employed as the magnetic powder in these particulate magnetic recording media, it is conceivable to employ the technique described in the Journal of the Magnetics Society of Japan 29, 239-242 (2005) to lower the switching magnetic field of the hexagonal ferrite magnetic material. However, the nonmagnetic organic material support that is normally employed in these particulate media has poor resistance to heat. Thus, it is difficult to employ the technique described in the Journal of the Magnetics Society of Japan 29, 239-242 (2005) in which the support is exposed to high temperatures during vapor phase film formation. Further, the present inventor studied the technique described in Japanese Patent No. 2,659,957. As a result, it was revealed that the crystalline structure of the hexagonal ferrite contributing to high thermal stability was deteriorated following reduction processing.
Accordingly, an aspect of the present invention provides for a magnetic particle that can be applied to particulate magnetic recording media and that has both high thermal stability and good recording properties.
The present inventor conducted extensive research into achieving the above-stated magnetic particle, resulting in the discovery that by heat-treating a hexagonal ferrite magnetic material in reducing atmosphere containing hydrocarbon gas, it was possible to adjust a coercive force to within a range suitable for recording while maintaining thermal stability. The present inventor surmised the reasons for this to be as follows.
When a hexagonal ferrite magnetic material is heat-treated in reducing atmosphere containing hydrocarbon gas, the hydrocarbon is oxidized as the hexagonal ferrite magnetic material is reduced. This produces carbon and/or carbides (which are referred to collectively as “carbon components” in the present invention). These are thought to deposit on the surface of the magnetic material. Since these carbon components are present on the surface of the magnetic material, the hexagonal ferrite magnetic material is not reduced completely through to the interior. As a result, a core/shell structure with hexagonal ferrite at the core and the reduction product thereof as the shell is thought to exist following the above heat-treatment in the reducing atmosphere. This core portion and shell portion are thought to exchange-couple. The spin orientation of the shell portion changes first in a manner corresponding to the change in the external magnetic field, thereby enabling a change in the spin orientation of the core portion with which the shell portion is exchange-coupled. It is thought that as a result, the switching magnetic field of the magnetic particle as a whole decreases (the coercive force decreases). However, the high thermal stability due to the crystalline structure of the hexagonal ferrite that has not been reduced in the interior of the particle can remain. The present inventor presumes that a magnetic particle that both exhibits high thermal stability and good recording properties is thus achieved.
By contrast, in an investigation conducted by the present inventor, in reducing atmosphere not containing hydrocarbon gas (for example, atmosphere containing carbon monoxide or the hydrogen atmosphere described in Japanese Patent No. 2,659,957), the reduction treatment caused the coercive force of the magnetic material to drop precipitously, making it difficult to obtain a magnetic material of a coercive force suited to high-density recording. This is thought to be because the hydrogen or carbon monoxide itself is oxidized when the hexagonal ferrite magnetic material is reduced, becoming water or carbon dioxide, respectively, and exiting the system. It is thought that, in the above system, a decomposition product is not formed on the surface of the hexagonal ferrite and thus reduction is not inhibited.
The present invention was devised based on the above discovery.
An aspect of the present invention relates to a magnetic particle, which is obtained by heat-treating a hexagonal ferrite magnetic material in reducing atmosphere containing hydrocarbon gas.
The above magnetic particle may be obtained by subjecting a hexagonal ferrite magnetic material having a coercive force of equal to or higher than 230 kA/m to the above heat-treatment.
The above magnetic particle may comprise a carbon component.
The above carbon component may be graphite.
The above reducing atmosphere may be mixed gas atmosphere of hydrocarbon gas and inert gas.
The above hydrocarbon gas may be at least one selected from the group consisting of methane and ethane.
The above magnetic particle may have a coercive force of equal to or higher than 120 kA/m but less than 230 kA/m.
The above magnetic particle may have thermal stability in the form of a gradient of decay of magnetization over time of equal to or less than 0.005 (1/ln(s)).
The above magnetic particle may have thermal stability in the form of a difference of a decay rate A and a decay rate B, B-A, ranging from 0.0001 to 0.001, wherein the decay rate A is measured by saturating magnetization of the magnetic particle with an external magnetic field of 40,000 Oe at a temperature of 300 K, subsequently changing the external magnetic field to −600 Oe, and measuring the decay rate based on a time at which a demagnetizing field reaches 600 Oe, and the decay rate B is measured by heating the magnetic particle of which the decay rate A has been measured to 320 K at a rate of temperature increase of 5° C./minute, maintaining the magnetic particle for 10 minutes at that temperature, subsequently cooling the magnetic particle to 300 K at a rate of temperature decrease of 5° C./minute, and measuring the decay rate by the same method as that of the decay rate A.
A further aspect of the present invention relates to a magnetic particle, which is comprised of a hexagonal ferrite magnetic material in which α-Fe and a carbon component are detected by X-ray diffraction analysis.
In the above magnetic particle, a quantity of α-Fe detected by X-ray diffraction may range from 0.2 to 1.0 weight percent.
The carbon component may be graphite.
The above magnetic particle may have a coercive force of equal to or higher than 120 kA/m but less than 230 kA/m.
The above magnetic particle may have thermal stability in the form of a gradient of decay of magnetization over time of equal to or less than 0.005 (1/ln(s)).
The above magnetic particle may have thermal stability in the form of a difference of a decay rate A and a decay rate B, B-A, ranging from 0.0001 to 0.001, wherein the decay rate A is measured by saturating magnetization of the magnetic particle with an external magnetic field of 40,000 Oe at a temperature of 300 K, subsequently changing the external magnetic field to −600 Oe, and measuring the decay rate based on a time at which a demagnetizing field reaches 600 Oe, and the decay rate B is measured by heating the magnetic particle of which the decay rate A has been measured to 320 K at a rate of temperature increase of 5° C./minute, maintaining the magnetic particle for 10 minutes at that temperature, subsequently cooling the magnetic particle to 300 K at a rate of temperature decrease of 5° C./minute, and measuring the decay rate by the same method as that of the decay rate A.
A still further aspect of the present invention relates to a method of preparing a magnetic particle, which comprises heat-treating a hexagonal ferrite magnetic material in reducing atmosphere containing hydrocarbon gas.
The above reducing atmosphere may be mixed gas atmosphere of hydrocarbon gas and inert gas.
The above hydrocarbon gas may be at least one selected from the group consisting of methane and ethane.
The above hexagonal ferrite magnetic material may be a hexagonal ferrite magnetic material having a coercive force of equal to or higher than 230 kA/m.
The above heat-treatment may yield a magnetic particle having a coercive force lower than that of the hexagonal ferrite magnetic material prior to the heat-treatment.
The above heat-treatment may yield a magnetic particle having a coercive force of equal to or higher than 120 kA/m but less than 230 kA/m.
The above heat-treatment may be conducted at a temperature ranging from 200 to 400° C.
A still further aspect of the present invention relates to a magnetic recording medium comprising a magnetic layer comprising a ferromagnetic powder and a binder, wherein the ferromagnetic powder comprises the above magnetic particle.
The present invention can provide a magnetic particle of high thermal stability and of a coercive force suited to recording that can be employed in particulate magnetic recording media.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure.